Lysosomal Storage Diseases
Lysosomal Storage diseases (LSD) are inherited genetic defects, resulting in an enzyme deficiency. This deficiency prevents the lysosome from metabolizing cellular waste, and results in their accumulation in the cell. Excessive storage of proteins, saccharides and/or fats can cause permanent cellular and tissue damage, particularly in the brain, peripheral nervous system, liver, spleen, and bone marrow.
In some of the diseases the enzyme may be present but there are defects in its transport into the lysosome. There are currently more than 45 known conditions, which fall within the LSD category.
The diseases produce a variety of symptoms leading to progressive physical and/or mental deterioration over time. Some of the diseases may be present in a “mild” form, and others have a more severe impact on the patient. A number of patients survive into adulthood, but others with more severe symptoms may die in their teens or earlier.
Late-Onset Tay-Sachs Disease (LOTS), a LSD variant of Tay-Sachs, is much less common than the infantile form of the disease. As the name suggests, LOTS affects adults rather than infants, and manifests itself as a progressive loss of function of the nervous system. The enzyme defect, a deficiency or malfunction of the alpha subunit of β-hexosaminidase A (HEXA) resides in the same peptide as that of classical Tay-Sachs, but people with the late-onset condition have some minor residual β-hexosaminidase A activity rather than a complete absence of the active enzyme.
The onset of symptoms in LOTS patients is usually between adolescence and the mid-30's, with much variation among individuals. Neurological manifestations of the disease include muscle weakness, cramping, wasting, and twitching; lack of coordination; slurred speech; and dystonia. Some LOTS patients have reduced intellectual functions, which may involve memory impairment, difficulty with comprehension and deterioration in school performance. Behavioral alterations can include short attention span and changes in personality. About 40% of LOTS patients exhibit psychiatric symptoms such as psychotic episodes or depression.
For an effective treatment of LSDs, a therapeutic agent, such as the deficient enzyme, must be taken up by the affected cells and routed to the lysosome where it is able to act upon the harmful material residing therein.
Blood-Brain Barrier
The blood-brain barrier (BBB) tightly regulates the transport of molecules into the brain, such that hydrophilic molecules whose size exceeds a molecular weight of 300-500 are usually prevented from access to the central nervous system. The BBB is formed by the microvasculature of the brain and permeability is regulated by the capillary endothelial cell. Overall, there are three types of recognized BBB transport systems:
Carrier-mediated transport (CMT); Active efflux transport (AET); and Receptor-mediated transport (RMT).
RMT systems have been used to by pass the BBB, in what has been termed the “Trojan horse” approach: one compound, which is recognized by the BBB as “friendly” and permitted to pass into the brain or actively transported into the brain (the “Trojan horse”) is conjugated or fused to a therapeutic molecule. This technique is useful for delivery of therapeutic proteins into the brain for the treatment of, for example, lysosomal storage disorders (LSD) and neurodegenerative diseases.
Certain therapeutic protein conjugates and/or fusion proteins are known in the art. US Patent Application Publication No. US 2005/0142141 relates to conjugates or fusion proteins composed of a therapeutic enzyme and a BBB targeting agent. The BBB targeting agent is referred to therein as a “molecular Trojan horse”. According to that patent application, the BBB targeting agent is selected from transferrin, insulin, leptin, insulin-like growth factors, cationic peptides, lectins, or peptidomimetic monoclonal antibodies directed to the transferrin, insulin or leptin receptors. The invention is exemplified by a recombinant fusion protein comprising humanized murine monoclonal antibody to the insulin receptor fused to α-L-iduronidase and a conjugate of the rat anti-mouse transferrin receptor monoclonal antibody to the β-galactosidase.
PCT Publication No. WO 2003/057179 teaches conjugates composed of p97 (melanotransferrin) covalently linked to an enzyme associated with LSD.
Pan et al (2004) report that the 39 kDa receptor-associated protein (RAP) may provide a novel means of protein-based drug delivery to the brain.
PCT Publication No. WO 89/10134 relates to chimeric peptides for neuropeptide delivery through the blood-brain barrier. The chimeric peptides comprise a neuropeptide and a peptide exemplified by histone, capable of crossing the blood-brain barrier via receptor-mediated transcytosis. The neuropeptides act on extracellular receptors to exert their therapeutic effects and do not enter the neural cells.
U.S. Pat. No. 6,027,720 teaches a modified polypeptide having human G-CSF activity. The novel polypeptides differ from native G-CSF as a result of a substitution or deletion in the N-terminus domain of the protein.
U.S. Pat. Nos. 6,555,660 and 6,831,158 relate to a modified G-CSF polypeptide and to conjugates thereof with a non-polypeptide moiety. The novel polypeptides differ from native G-CSF in at least one amino acid residue, which is replaced by an amino acid having an attachment group for a polymer.
U.S. Pat. No. 6,518,235 relates to a method of improving memory in a patient comprising the administration of leptin.
There remains an unmet need for therapeutic agents, which are able to cross the blood brain barrier, for the treatment of lysosomal storage diseases and neurodegenerative disorders.